In certain situations, workers in the oil and gas industry perform a procedure known as "hydraulic fracturing" during oil exploration and drilling operations. For example, in formations where oil or gas cannot be easily or economically extracted from the earth, a hydraulic fracturing operation is commonly performed. Such a hydraulic fracturing operation includes pumping in large amounts of fluid to induce cracks in the earth, thereby creating pathways via which the oil and gas may flow. After a crack is generated, sand or some other material is commonly added to the crack, so that when the earth closes back up after the pressure is released, the sand helps to keep the earth parted. The sand then provides a conductive pathway for the oil and gas to flow from the newly formed fracture.
However, the hydraulic fracturing process does not always work very well. The reasons for this are relatively unknown. In addition, the hydraulic fractures cannot be readily observed, since they are typically thousands of feet below the surface of the earth. Therefore, members of the oil and gas industry have sought diagnostic methods to tell where the fractures are, how big the fractures are, how far they go and how high they grow. Thus, a diagnostic apparatus and method for measuring the hydraulic fracture and the rock deformation around the fracture are needed.
In previous attempts to solve this problem, certain methods have been developed for mapping fractures. For example, one of these methods involves seismic sensing. In such a seismic sensing operation, micro-earthquakes generated by the fracturing are analyzed by seismic meters, for example, accelerometers.
A second known type of fracture mapping involves running analytical models which are based only on pressure measurements taken at the fracture region. Another method of attempting to map the fractures is performed by using radioactive materials. The radioactive materials are inserted into the earth, and then radioactive counters are used to determine where the radioactive materials are to help in the determination of the geometry of the fracture.
Surface tiltmeter arrays have also been used in assessing hydraulic fracture geometry. Tiltmeters operate like a level in that they can detect any deviation or tilt in position from horizontal that may be caused by the hydraulic fracturing process. By definition, surface tiltmeter arrays are located only on the surface of the earth. This causes a major problem in that the signals acquired at the surface array are extremely small since they have to pass through approximately 5,000-10,000 feet of earth before reaching the surface where the meter is located.
Tiltmeters are sensitive instruments for measuring an angular rotation within a ground mass. Tiltmeters have been employed for many years in monitoring the deformation of ground masses due to earthquakes, volcanism, earth tides, and many underground processes associated with resource extraction and waste disposal. In such situations, tiltmeters have been employed in various linear and areal arrays, usually on the surface of the earth, but occasionally in mines or other underground openings very near the surface. Tiltmeters can be packed in place with sand or cemented in place permanently.
Tiltmeters can be designed in many ways, but the most sensitive commercial instruments use bi-axial electrolytic tilt sensors to measure the tilt in two orthogonal vertical planes. Such devices can measure rotations with nanoradian sensitivity and accuracy (1 nanoradian=5.7.times.10.sup.-8 degrees=0.0002 arc sec).
The arrays are normally deployed at surface locations in shallow sand-filled holes, e.g., 10 ft. to 30 ft. deep. Typical deployment occurs in an extended elliptical or circular geometries surrounding the expected surface projection of the fracture azimuth. The composite data from this horizontally deployed array is inverse modeled to provide the engineer with a most probable set of fracture parameters, such as dip and fracture azimuth, related to the interpretation of measured deformation.
As set forth above, the surface tilt array for hydraulic-fracture mapping is generally circular, but may have additional sensors at strategic locations. Tiltmeters are usually put in shallow holes (10-30 ft.) to minimize surface noise due to the solar heating and cultural causes and to avoid as much of the unconsolidated surface layers as possible. During a hydraulic fracture at depth (usually depths up to 5000 ft can be monitored with tiltmeters), the ground experiences a slight rotation due to the expanding fracture that will vary with location around the fracture plane. These rotations are measured by the tiltmeter, and models of the expected behavior are compared with the tilt results to estimate fracture characteristics. However, tilts at the surface have large fluctuations due to tides, heating, and cultural sources that need to be subtracted out in order to properly estimate the fracture response.
However, all the above methods may or may not work, and when they do it is with varying degrees of success. As a result, a different approach utilizing a more effective apparatus and method of diagnostic monitoring is needed.
Therefore, as explained above, a great need exists, especially in the oil and gas industry, as well as other fields, for a diagnostic apparatus and method that monitors hydraulic fracture dimensions to provide useful data on the deformations of the rock caused by the fractures.